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grouegroue
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Delete the rationale for Sendable records
... because the reality is that I just don't know how to relax this requirement. See: - https://www.youtube.com/watch?v=JmrnE7HUaDE - https://forums.swift.org/t/swiftserverconf-feedback-on-leveraging-structured-concurrency-in-your-applications/75176
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‎GRDB/Documentation.docc/SwiftConcurrency.md‎

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@@ -39,11 +39,73 @@ All safety guarantees of Swift 6 are enforced during database accesses. They are
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The following sections describe, with more details, how GRDB interacts with Swift Concurrency.
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- <doc:SwiftConcurrency#Non-Sendable-Record-Types>
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- <doc:SwiftConcurrency#Shorthand-Closure-Notation>
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- <doc:SwiftConcurrency#Non-Sendable-Configuration-of-Record-Types>
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- <doc:SwiftConcurrency#Non-Sendable-Record-Types>
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- <doc:SwiftConcurrency#Choosing-between-Synchronous-and-Asynchronous-Database-Accesses>
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### Shorthand Closure Notation
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In the Swift 5 language mode, the compiler emits a warning when a database access is written with the shorthand closure notation:
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```swift
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// Standard closure:
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let count = try await writer.read { db in
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try Player.fetchCount(db)
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}
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// Shorthand notation:
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// ⚠️ Converting non-sendable function value to '@Sendable (Database)
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// throws -> Int' may introduce data races.
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let count = try await writer.read(Player.fetchCount)
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```
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**You can remove this warning** by enabling [SE-0418: Inferring `Sendable` for methods and key path literals](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0418-inferring-sendable-for-methods.md), as below:
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- **Using Xcode**
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Set `SWIFT_UPCOMING_FEATURE_INFER_SENDABLE_FROM_CAPTURES` to `YES` in the build settings of your target.
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- **In a SwiftPM package manifest**
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Enable the `InferSendableFromCaptures` upcoming feature:
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```swift
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.target(
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name: "MyTarget",
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swiftSettings: [
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.enableUpcomingFeature("InferSendableFromCaptures")
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]
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)
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```
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This language feature is not enabled by default, because it can potentially [affect source compatibility](https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/sourcecompatibility#Inferring-Sendable-for-methods-and-key-path-literals).
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### Non-Sendable Configuration of Record Types
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In the Swift 6 language mode, and in the Swift 5 language mode with strict concurrency checkings, the compiler emits an error or a warning when a record type specifies which columns it fetches from the database, with the ``TableRecord/databaseSelection-7iphs`` static property:
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```swift
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extension Player: FetchableRecord, PersistableRecord {
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// ❌ Static property 'databaseSelection' is not concurrency-safe
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// because non-'Sendable' type '[any SQLSelectable]'
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// may have shared mutable state
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static let databaseSelection: [any SQLSelectable] = [
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Column("id"), Column("name"), Column("score")
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]
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}
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```
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**To fix this error**, replace the stored property with a computed property:
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```swift
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extension Player: FetchableRecord, PersistableRecord {
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static var databaseSelection: [any SQLSelectable] {
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[Column("id"), Column("name"), Column("score")]
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}
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}
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```
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### Non-Sendable Record Types
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In the Swift 6 language mode, and in the Swift 5 language mode with strict concurrency checkings, the compiler emits an error or a warning when the application reads, writes, or observes a non-[`Sendable`](https://developer.apple.com/documentation/swift/sendable) type.
@@ -128,97 +190,6 @@ You do not need to perform this refactoring right away: you can compile your app
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Yes. Classes that can not be modified, made of constant `let` properties, are Sendable. Those immutable classes will not make it easy to modify the database, though.
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#### FAQ: Why this Sendable requirement?
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**GRDB needs new features in the Swift language and the SDKs in order to deal with non-Sendable types.**
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[SE-0430: `sending` parameter and result values](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0430-transferring-parameters-and-results.md) looks like the language feature we need, but:
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- `DispatchQueue.async` does not accept a `sending` closure. GRDB needs this in order to accept non-Sendable records to be sent to the database, as below:
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```swift
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let nonSendableRecord: Player
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try await writer.write { db in
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try nonSendableRecord.insert(db)
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}
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```
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Please [file a feedback](http://feedbackassistant.apple.com) for requesting this DispatchQueue improvement. The more the merrier. I personally filed FB15270949.
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- Database access methods taint the values they fetch. In the code below, the rules of [SE-0414: Region based Isolation](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0414-region-based-isolation.md) have the compiler refuse that the fetched player is sent back to the caller:
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```swift
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let player = try await writer.read { db in
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try Player.fetchOne(db, id: 42)
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}
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```
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Strictly speaking, the compiler diagnostic is correct: one could copy the non-Sendable `db` argument into the fetched `Player` instance, making it unsuitable for later use. In practice, nobody does that. Copying `db` is a programmer error, and GRDB promptly raises a fatal error whenever a `db` copy would be improperly used. But there is no way to tell the compiler about this practice.
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For all those reasons, GRDB has to require values that are asynchronously written and read from the database to be `Sendable`.
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### Shorthand Closure Notation
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In the Swift 5 language mode, the compiler emits a warning when a database access is written with the shorthand closure notation:
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```swift
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// Standard closure:
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let count = try await writer.read { db in
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try Player.fetchCount(db)
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}
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// Shorthand notation:
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// ⚠️ Converting non-sendable function value to '@Sendable (Database)
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// throws -> Int' may introduce data races.
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let count = try await writer.read(Player.fetchCount)
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```
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**You can remove this warning** by enabling [SE-0418: Inferring `Sendable` for methods and key path literals](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0418-inferring-sendable-for-methods.md), as below:
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- **Using Xcode**
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Set `SWIFT_UPCOMING_FEATURE_INFER_SENDABLE_FROM_CAPTURES` to `YES` in the build settings of your target.
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- **In a SwiftPM package manifest**
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Enable the `InferSendableFromCaptures` upcoming feature:
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```swift
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.target(
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name: "MyTarget",
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swiftSettings: [
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.enableUpcomingFeature("InferSendableFromCaptures")
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]
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)
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```
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This language feature is not enabled by default, because it can potentially [affect source compatibility](https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/sourcecompatibility#Inferring-Sendable-for-methods-and-key-path-literals).
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### Non-Sendable Configuration of Record Types
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In the Swift 6 language mode, and in the Swift 5 language mode with strict concurrency checkings, the compiler emits an error or a warning when a record type specifies which columns it fetches from the database, with the ``TableRecord/databaseSelection-7iphs`` static property:
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```swift
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extension Player: FetchableRecord, MutablePersistableRecord {
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// ❌ Static property 'databaseSelection' is not concurrency-safe
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// because non-'Sendable' type '[any SQLSelectable]'
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// may have shared mutable state
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static let databaseSelection: [any SQLSelectable] = [
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Column("id"), Column("name"), Column("score")
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]
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}
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```
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**To fix this error**, replace the stored property with a computed property:
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```swift
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extension Player: FetchableRecord, MutablePersistableRecord {
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static var databaseSelection: [any SQLSelectable] {
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[Column("id"), Column("name"), Column("score")]
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}
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}
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```
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### Choosing between Synchronous and Asynchronous Database Accesses
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GRDB connections provide two versions of `read` and `write`, one that is synchronous, and one that is asynchronous. It might not be clear how to choose one or the other.

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